As data are transmitted through telephone or wireless channels, the receiver must be able to detect the timing information in the received signal to synchronize its clocks to the data. A conventional timing recovery system on the receiver's end for use in connection with data communication is shown in FIG. 1. The received signal 10 is first applied to a nonlinearity operator, such as the squarer 12. The output from the square 12 is then filtered by a narrow-band filter or a phase-locked loop 14 to retrieve the desired timing signal. The recovered timing signal can thus to be used to facilitate the sampler on the receiver's end to receive data. The conventional system generally fails to optimize the timing phase.
FIG. 2 illustrates another conventional timing recovery system, commonly referred to as the "Early-Late" system, for optimizing the timing phase and frequency for synchronizing data receivers. A received signal 21 from a sampler 20 has samples in different phases (represented by Phase A and Phase B). The signal in one phase is filtered by a half-symbol-rate band-pass filter 22 to produce an EARLY sample, while the signal in another phase is filtered by another half-symbol-rate band-pass filter 23 to produce a LATE sample. The filtered signals are then applied to a nonlinearity operator, such as the squarers 24, 25, to generate their respective symbol components. A difference between the squared outputs is generated by an adder 26 as an error signal.
The error signal can then be applied to a timing phase-locked loop 27 to recover a timing signal 28 for the sampler 20 to synchronize the received signal. When the received signal is synchronized, the error signal should be zero. When such a situation occurs, the phase-locked loop 27 will settle to one unique phase, which is thus the optimal timing sampling phase for a T-spaced equalizer. This technique is commonly called the "band-edge component maximization" technique in that it uses the band-edge energy in the received signal to track timing. The general goal is to keep the two samples, EARLY and LATE, roughly equal to derive their optimal sampling time, which is generally half-way between the two samples.
There are problems associated with the conventional "Early-Late" technique. Because information at the band-edge is often heavily attenuated by the channel, there is very little energy, i.e. little information, at the band-edge for the timing circuit. The tracking range of the phase-locked loop depends on the channel, e.g. too high for high bandedge gain and too low for low gain.